Does copying jpeg files reduce the quality of their files at any given time, and eventually make small changes to the size of the copied files compared to the original file ?
The degeneration one would have to open and resave the file 10,000 times. Your not going to lose any image quality the few hundred times you open and close the file, and or, edit the file. I use photofiltre and can save at 100% file. Anyways, if you're that worried about it, always edit with the image converted to tiff and or a 2nd copy jpg of the original by "Save As" and renaming the copy. Tiff is a lossy compression huge file (Larger than a RAW file), so edits will take somewhat longer to perform. Tiff is uncompressed and won't compress. Never edit and resave the original jpg. Open the original jpg and resave as a tiff and close the jpg, and open the tiff for editing. Using the tiff for editing, this will limit the corruption of the jpg file as the original jpg wont be opened or used much over time, only opened to edit. Merely opening and closing a jpg won't affect it. Always work on the tiff file and save that file as a renamed jpg, saving renamed, this way your jpg will always be a fresh new saved jpg. for re-edits when you open and save the new jpg to tiff, once saved as a tiff you can delete the jpg as you will save the re-edited tiff as a new jpg, no degeneration as you will be saving the edited tiff as a "new fresh jpg". No loss of file when opening, no loss if file when saving as new file name as the file will be a fresh save file and not rewritten file. It's when files are rewritten info is lost. rewritten is when edited and saved as original file name. if you change the file name it wont rewrite over the original.
Basically open the original jpg for the first time for editing. Open the original, save to tiff or save copy as, and close the original. Edit the tiff or the saved as jpg, leaving the original closed and save the tiff/jpg after editing as a new file name jpg. if you want to edit in jpg open the original and resave as another file name and close the jpg. Example: original jpg file (DSC3400.jpg) Save as (DSC34e00.jpg) Notice the "e" stands for edited, after the new file name is saved close the original and open and edit the 2nd file with the letter e added to the name, after editing save that file and rewrite over it or rename it again for a fresh jpg. This way you wont be re-writing over the original.
File copy is a lossless operation. Disks usually hace CRC checks in place to detect if a sector is corrupted but the act of copying is a 1:1 bit copy, so each copy is exactly identical to the previous.
The loss of quality occurs on compression when a file is written from image-data, JPEG encoding discards some information, and even so JPEG supports certain lossless operations, allowing a file to be changed in a way that does not occur a loss of quality, for example rotation by 90 degrees.
When you see examples of JPEG degradation, it is usually that someone opens a JPEG and saves it again after modification or creates a new JPEG by pasting the contents of another one in it and saves repeatedly. This will cause degradation because lossy compression occurs each time.